Fuel cells were invented in 1839 by Sir William Grove. A fuel cell is an electrochemical device which directly combines a fuel such as hydrogen and an oxidant such as oxygen to produce electricity and water. It has an anode and a cathode spanned by an electrolyte. Hydrogen is oxidized to hydrated protons on the anode with an accompanying release of electrons. At the anode, oxygen reacts with protons to form water, consuming electrons in the process. Electrons flow from the anode to the cathode through an external load, and the circuit is completed by ionic current transport through the electrolyte.
Fuel cells do not pollute the environment. They operate quietly, and they have a potential efficiency of ca. 80 percent. Virtually any natural or synthetic fuel from which hydrogen can be extracted--by steam reforming, for example--can be employed.
A variety of electrolytes have been proposed. These include: aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonates, and stabilized zirconium oxide. Molten carbonate fuel cell (MCFC) power plants are of particular interest. A MCFC power plant, for example, offers cost savings and increased efficiency in converting natural gas to electrical energy in comparison to other available techniques for accomplishing this goal such as using this abundantly available gas to fuel a gas turbine engine (potential conversion efficiency of 30%). Because of cost, performance, and endurance considerations, the basic components of a MCFC fuel cell must be: easily manufactured by simple scalable techniques, stable in the fuel cell, and able to meet threshold performance levels.
Molten carbonate fuel cell (MCFC) components are thin, flat materials with a porosity and pore size distribution carefully tailored for proper electrolyte distribution by virtue of capillarity. The basic cell package typically consists of an anode, a cathode, one or more bubble barriers, and an electrolyte structure composed of a matrix and an electrolyte retained in the matrix by capillarity.
Present state-of-the art production processes for manufacturing MCFC components include tape casting and dry, loose packing/sintering of component powders.
Tape casting, a "wet" process, involves the mixing of component powders with a liquid binder which contains additives custom tailored to impart desired properties to the powder/binder suspension. This mixture is then "casted" into a uniform tape via a "doctor blade" which strikes across the slurry surface at controllable heights. These tapes are dried and, depending on the casted component, further processed through pore-size-fix rolling, binder burnout, sintering, thickness-set-rolling, and annealing steps.
The "loose packing" method involves the spreading of dry powders within a recessed die and "striking" the top powder surface with a straight-edge. This method is still used by some developers for making electrodes. Like tape casting, subsequent processing steps may include sintering, thickness-set-rolling, and annealing.
As MCFC technology moved into the commercialization phase of the development process, the target active area for MCFC components increased from 16 in.sup.2 to 1 ft.sup.2 to up to 10 ft.sup.2. This increase in size has brought to light the scalability of the present porous component manufacturing techniques, particularly with regard to quality control and subsequent handleability and ease of assembly. State-of-the-art manufacturing techniques are at best "pushed" to attain and assure uniform thickness, surface, and microstructural tolerances and strength through numerous drying, rolling, and sintering steps. Additionally, tape and dry-loose casting methods presently require that as many as six to ten large pieces be assembled to stringent fit-up tolerances to make a single cell. Typically the cathode, anode, and anode bubble barrier will each be fabricated from two pieces of material and the electrolyte structure from three pieces.